Add Config.read_config/1 (#13754)

lib/elixir/lib/config.ex

@@ -194,6 +194,31 @@ defmodule Config do
     |> put_config()
   end
 
+  @doc """
+  Reads the configuration for the given root key.
+
+  This function only reads the configuration from a previous
+  `config/2` or `config/3` call. If `root_key` points to an
+  application, it does not read its actual application environment.
+  Its main use case is to make it easier to access and share
+  configuration values across files.
+
+  If the `root_key` was not configured, it returns `nil`.
+
+  ## Examples
+
+      # In config/config.exs
+      config :my_app, foo: :bar
+
+      # In config/dev.exs
+      config :another_app, foo: read_config(:my_app)[:foo] || raise "missing parent configuration"
+
+  """
+  @doc since: "1.18.0"
+  def read_config(root_key) when is_atom(root_key) do
+    get_config!()[root_key]
+  end
+
   @doc """
   Returns the environment this configuration file is executed on.
 

lib/elixir/test/elixir/config_test.exs

@@ -64,6 +64,16 @@ defmodule ConfigTest do
     assert config() == [app: [{Repo, other: :value, key: :other}]]
   end
 
+  test "read_config/1" do
+    assert read_config(:lager) == nil
+
+    config :lager, key: :value
+    assert read_config(:lager) == [key: :value]
+
+    config :lager, other: :value
+    assert read_config(:lager) == [key: :value, other: :value]
+  end
+
   @tag env: :dev
   test "config_env/0" do
     assert config_env() == :dev